1. Field of the Invention
This invention relates to ion sources.
An ion source is an apparatus which produces ions in ion accelerating apparatus which uses these ions. Such an ion source ionizes atoms of a material necessary for some specific purpose, and the ion accelerating apparatus accelerates the ions using an electric field, etc.
One type of ion accelerating apparatus used in industry is ion implanting apparatus which is used to manufacture semiconductor devices. In such apparatus, in order to form P-N junctions on silicon wafers, one makes use of the production of various ions by means of an ion source, such as boron (B), phosphorus (P), arsenic (As), or antimony (Sb). Such ions are accelerated by any of a number of various ion accelerators, such as single-stage accelerators, tandem accelerators, rf linear accelerators, etc.
Among the aforementioned ions, only boron can be used as a P type dopant.
2. Description of the Prior Art
In order to produce these boron ions, since boron itself has a very high melting point of 2300.degree. C., it is difficult to produce the vapor, and in the past mainly BF.sub.3 (on rare occasions BCl.sub.3) have been used as the material for supplying the ion source. However, when these molecular-condition materials are supplied to the ion source, various types of ions such as F.sup.+, BF.sup.+, BF.sub.2.sup.+, etc. are formed in addition to the desired B.sup.+, and the defect occurs that the yield of the desired ion is adversely affected. Moreover, in order to increase the yield of B.sup.+ (viz. the rate of decomposition of molecules of BF.sub.3, etc.), one raises the temperature of the plasma, and it becomes necessary to use a greater scale filament electric power supply, anode electric power supply, cooling system, etc. Thus the defect occurs that the apparatus becomes large scale and high price. Moreover, electric discharges, etc. occur frequently because of higher power consumption, and thus the defect occurs that the operation of the ion source becomes unstable.